Neville MADDEN <[email protected]> writes: > Good morning, I have OSM And+ with the latest update on a Lenovo Idea Pad > Pro Tablet. On a recent trip to India the altimeter seems to be reading > about 50m below known altitudes. (All Indian Railway stations have their > Reduced Level (RL) on the station approach sign). Is there any way I can > re-calibrate and adjust the altimeter?
GNSS receivers do not actually have an "altimeter". They compute a position, which includes "height above ellipsoid", often written h. Then, they convert this, using a "geoid model", to "height above geoid", often written H. Generally, H corresonds to what people mean when they say "altitude above mean sea level". (I say "what they mean" because really understanding height is tremendously complicated.) In OsmAnd, there is a data file "World Altitude Corrections", that if you download, you should see heights/elevations displayed as H. If you don't have that, you get h. For reasons that are unclear to me, this is marked Android only. India has pretty high geoid separations -- the difference between geoid and ellipsoid, written N. It looks like -68m at Mumbai from a quick look. So this could explain your displayed height values. If anyone understands what happens on iOS, please share. It's possible that a model built in to iOS is used. GNSS receivers typically have builtin models, but also typically they are extremely poor. For example around me the real value is roughly -29m and the models say -33m. Most programs that use GNSS output (on android or normal computers) will back out the receivers model. NMEA reports H and N, and then one can compute h, which avoids the terrible model. You can then apply a better model. Really - it's that messy. -- You received this message because you are subscribed to the Google Groups "OsmAnd" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion visit https://groups.google.com/d/msgid/osmand/rmiwm0lxyvr.fsf%40s1.lexort.com.
